In recent years, a system has been practiced which includes a serial tandem type image forming system which carries out duplex printing, or the like, by connecting a plurality of image forming apparatuses in series. More specifically, in a case of a serial tandem type image forming system, in which two image forming apparatuses are connected in series, a toner image is formed on the front surface of a paper sheet via an image forming apparatus (preceding stage apparatus), arranged in a preceding stage, and a toner image is formed on the back surface of the paper sheet via an image forming apparatus (subsequent stage apparatus), arranged in a subsequent stage. In such a way, the printing speed can be improved, when compared with a case in which only one image forming apparatus carries out the duplex printing.
An image forming apparatus, which is equipped with a maintenance counter for determining the target parts for maintenance (the parts subject to maintenance), has been disclosed in Unexamined Japanese Patent Application Publication No. 2001-56627, as an example. In that image forming apparatus, a predetermined additional value is added to the count value of a maintenance counter in response to formation of a toner image on a paper sheet. In this case, the additional value is set to a value corresponding to the amount of paper dust generated from paper sheets onto which toner images are formed, and thus, the additional value is set to a larger value in cases in which the amount of generated paper dust is larger.
Meanwhile, an image forming apparatus which carries out additional printing to form a toner image on a paper sheet (pre-printed paper sheet), on which an image has been formed in advance, has been disclosed in Unexamined Japanese Patent Application Publication No. 2008-83302, as an example. According to the disclosure, vapor evolved from the preliminary formed images and compositions which form the images adheres onto the respective sections of the image forming apparatus, resulting in contamination of the apparatus, which prevents formation of an excellent image.
By the way, in the case of a serial tandem type image forming system, even in a case in which an individual image forming apparatus operates independently in response to formation of toner images on a single paper sheet, there is a probability that a maintenance cycle, with respect to the same target parts for maintenance, may vary from one image forming apparatus to another image forming apparatus. As an example, in the case of an operation mode of an image forming system in which pre-printed paper sheets are used, the level of contamination, caused by the pre-printed paper sheets, may vary from one image forming apparatus to another, and therefore, such a probability may be particularly possible. Therefore, in a case of a serial tandem type image forming system, the time of maintenance (the time when maintenance needs to be carried out) may not be determined appropriately. In such case, there exist problems that image forming performance may deteriorate, and/or apparatus downtime associated with unnecessary maintenance may occur.